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To understand understeer and oversteer, we first must look
at the contact patch, i.e. were the tire is meeting the road,
and not think of that meeting in static terms. It is in three
dimentional motion, I'm sure even that is a over simplification,
but not only is it rolling over the ground, but deflecting
left/right, and twisting at some angle relative to the steel
wheel and suspension components, this twisting is known as "slip
angle". Don't be confused by the turm slip angle, the tire is
still twisting, not so much "sliping", if we have gone to really
slipping over the ground we have exceedied the coefficent of
friction, and the tire is no longer doing its job.
As a car go through a corner, centrifugical forces transfer
the center of mass around the center of gravity generating more
load on the outside tires. As the driver inputs steering input
by turning the steering wheel the desired change of direction is
tranmitted to the road wheel (through whatever linkage the
manufactuer designed) and generates twisting forces to the tire.
This difference of angle between the road wheel, and the
rubber tire is slip angle. The greater the vertical load, the
higher the coefficient of friction the greater the slip angle.
Now understand this is a very simple model we are building here,
believe me the permeatations of this situation, will send mere
mortals for the schnapps bottle.
If the front tires generate more "slip angle" than the rear
tires i.e. the front tires are on a larger radi than the rear,
the front of the car will be on a arc leading to the outside of
the turn i.e. understeer, if the rears have the larger slip
angle the rear tires are on a larger radi and so we have
Now simple tuning of the system with "sway bars", or anti
sway bars. The larger bars increase vertical loading of the
contact patch, increasing coefficient of friction, making the
slip angle larger. So which ever end of the car is miss
behaving, i.e. you want to cure understeer, you would put a
larger dia. bar in the rear, or shorten up the attachment point
and adjustment links on your fancy Weltmeister rear bar.
Shocks (kinetic energy absorbers), and spring are other
ways of tuning the car, even to the point changing bound v
rebound to control weight transfer through out the chassis at
different phases of a turn (enterance, middle, exit) Power down
thru the exit phase will produce a change in weight distribution
changing the slip angles again.
Tire pressure has some control over slip angle also, the
more pressure the higher the spring rate of the tire itself and
the more resistant to twisting it becomes, atleast to a point.
Then we run the risk of really elevating the tires temp and have
the potental performance degrade.
I can't recommend any of Carol Smith books to highly. They
are the best, most of this data can be found in "Tune to Win".
His other books are "Engineer to Win", "Prepare to Win". You
would not believe at SCCA race weekends how many tool boxs have
copies of these books in them.
Sorry this was long, just not a subject to lend itself to
'82 911 SC
'83 Spec Rx-7 (for sale, anyone want a turn key race car?)
looking for a 944 for PCA group I